The cells of the terminal end bud (TEB) are the most proliferative and invasive cells found during postnatal development in mammals. These cells, through their enormous proliferative capacity, drive the development of the entire mammary ductal system, serve as a source of mammary progenitor cells, and are the single most important target for carcinogens within the mammary gland. The ability of endocrine-, paracrine- and extracellular matrix-derived signals to harness this enormous regenerative potential and direct it into a highly ordered developmental pattern makes the TEB both an important and very fascinating system with which to study the endocrine regulation of intracellular signaling. Our analysis of grafted mammary tissue from a knockout mouse model has demonstrated that a targeted inactivating mutation of the gene for the IGF-I receptor (lgf1r) dramatically reduces mammary ductal development and decreases TEB cell proliferation(1). This proliferative defect, however, is partial restored by early pregnancy. In breast cancer cell models the IGF-I receptor as well as a handful of other hormone receptors signal through insulin receptor substrate (IRS) proteins, and two serine threonine kinases, ERK and Akt. The overall hypothesis addressed in this proposal is that IGF-I receptor stimulates TEB cell proliferation through insulin responsive substrate (IRS)-dependent activation of ERK and Akt during virgin ductal development and that pregnancy or the ovarian steroid hormones estradiol (E2) and progesterone (P) increase the sensitivity of IRS-dependent signaling pathways to insulin receptor-dependent activation in Igf1r-/- TEB. Through the use confocal microscopy with recently developed antibodies to detect cell signaling in-situ, we will learn how signaling pathways are regulated in TEBs and what the significance of these pathways is to processes important to both normal mammary gland function and breast cancer. The specific aims are; 1) determine if estrus cycle, ovarectomy, early pregnancy, or exogenous E2+P alters IGF-I-, or insulin-induced phosphorylation of IRS-1, -2, ERK and Akt in mammary glands of normal mice, 2) determine if IGF-IR or IR dependent activation of ERK, and Akt is attenuated in the TEB of lgf1r -/- grafts and if early pregnancy or exogenous E2+P can restore this activation, and 3) determine if overexpression of IRS-1 or IRS-2 within the mammary epithelium restores development of Igf1r-/- mammary epithelium and enhances insulin-dependent posphorylation of ERK and Akt.